Wireless communication systems are utilized to facilitate communication between a wireless mobile unit and other wireless mobile units as well as between wireless mobile units and customers of wired units, such as those attached to a public switched telephone network (PSTN). Wireless communication systems operate according to communication protocols within allocated frequency bands and on particular radio frequency (RF) channels contained within the frequency bands. Frequency bands currently in use in North America and around the world for wireless communications include the 800 MHz (cellular) band and the 1900 MHz (personal communication services (PCS)) band among others.
Various standardized communication protocols have been developed for wireless communication systems. These standardized protocols include, for example, the Advanced Mobile Phone Service (AMPS) standards, the Global Standards for Mobility (GSM), the Code Division Multiple Access (CDMA) and the Time Division Multiple Access (TDMA) standard. These standards support interoperability of equipment manufactured by differing vendors.
Wireless communication systems are generally constructed according to a hierarchy in which mobile switching centers (MSCs) (or mobile telephone exchanges (MTXs)) reside atop the hierarchy. The MSCs couple to the PSTN and also typically couple to packet data networks (PDNs) such as the Internet via an interworking function. In this hierarchy, each MSC (or MTX) services a plurality of base station controllers (BSCs) each of which services a plurality of base stations. The base stations establish wireless links with serviced mobile units. In this hierarchy, all calls (voice or packet data communications) are routed through the MSC (or MTX).
Cellular telephones provide convenience and safety to customers by giving customers the ability to make and receive telephone calls from any location where cellular services are available. The ability to receive telephone calls is limited to the times when the telephone is turned on. When the telephone is turned off, a caller will normally receive an announcement provided by the cellular service provider, or carrier, that the customer is currently unavailable or is not within a service area. With some systems, a caller calling when a customer's telephone is turned off will be able to leave a voice message for the customer. The customer can then retrieve such a message by calling into the service provider. Currently, a caller cannot tell if a customer's telephone is turned on and connected to the mobile network without calling the customer's telephone number.